1. Field of the Invention
The present invention relates to a motor system, more particularly to a motor system with reduced common-mode noise.
2. Description of Prior Art
FIG. 1 shows a power source scheme for a motor driving system. An AC power is converted by a transformer unit 42, which also comprises a rectifier and a DC-DC converter, to power a DC power source 44. The DC power source 44 supplies power to IGBT (Insulated Gate Bipolar Transistor) driving section and control section.
FIG. 2 shows the prior art motor system with driving section and control section, which are powered by a DC power 44A from the DC power source 44. The DC power 44A supplied by the transformer unit 42 is applied to the control section composed of a micro controller 50, an application specific integrated circuit (ASIC) 52 and an interface unit 54. Moreover, the DC power 44A is also applied to a photo coupler switch 56 and a power unit 58 (these two elements belong to driving section). The photo coupler switch 56 is controlled by the micro controller 50 to turn on or turn off a gate switch (not shown) in the power unit 58. Moreover, a safe stop signal 64 controls a relay switch 62 to cut off power to a motor 60.
However, the circuits shown in FIGS. 1 and 2 have following drawbacks. The motor system uses a single transformers to supply power to the power unit 58 and control section (includes the micro controller 50, the ASIC 52 and the interface unit 54 enclosed by the dashed line). The control section cannot get pure feedback signal (signal immune to noise). In other word, the two power sources (driving power source and the control power source) share the same transformer, the control power source is contaminated by the noise from the driving source, which is coupled through the transformer. The common mode noise is coupled to the control unit through the winding of the transformer and the noise will influence control function.
Moreover, the relay switch 62 for outputting U, V, W phase power is bulky and needs frequently maintenance. The safe stop signal 64 needs to control the relay switch 62 to turn off power to the motor 60. The circuit is complicated and has safety concern.